Portable base station network for local differential GNSS corrections

ABSTRACT

A DGNSS-based guidance system, wherein a rover receiver first utilizes data from a master base station transceiver, a DGNSS reference network, or some other differential source to compute a differentially corrected location to establish a reference DGNSS relationship. Using this location and data observed only at the rover, the rover computes an internal set of differential corrections, which set is stored in computer memory, updated as necessary, and applied in future times to correct observations taken by the rover. As the rover enters into areas of other base station receiver reference networks, the rover transceiver will send positional information it receives from the master base station to the new, secondary base station. The secondary base station then calibrates its own reference information using information sent from the original master base station.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/457,987, filed Aug. 12, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/300,609, filed Nov. 20, 2011, now U.S. Pat. No.8,803,735, issued Aug. 12, 2014, which claims priority in U.S.Provisional Patent Application Ser. No. 61/415,693, filed Nov. 19, 2010,all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Global Navigation SatelliteSystem (GNSS) base stations, and in particular to calibrating a networkof portable, secondary base or reference stations in relation to amaster base station for differential GNSS correction over a coverageregion comprising multiple overlapping coverage polygons.

2. Description of the Related Art

Differential GNSS techniques have been successfully applied for a numberof years. These techniques, for example, enable accurate real-timepositioning of a rover receiver relative to a base receiver. Thispositioning includes code-only or carrier-smoothed-code differentialtechniques that result in sub-meter accuracy, such as those employedwhile operating with older RTCM 104 messages. They include carrier phasebased techniques that facilitate centimeter-level real-time kinematic(RTK) positioning.

Differential GNSS (DGNSS), as its name implies, requires that data bedifferenced. One of the most useful differences in DGNSS, and thereforea widely used difference, is that of differencing two similarobservations of satellite ranging signals where one observation is madeat a base or reference GNSS receiver and another is made at a rover GNSSreceiver. This type of difference, commonly referred to as thesingle-difference, removes common-mode errors (i.e. errors seen by bothbase and rover receivers) such as satellite orbit errors, satelliteclock errors, and atmospheric errors that arise as the signal passesthrough the ionosphere and the troposphere. The remaining sources oferror that result when employing single-difference techniques are thosethat are unique to the receiver, such as receiver noise and multipath.These remaining errors are often small in comparison to the common-modeerrors, especially when carrier-phase observations are employed. Leftunchecked, these errors can, over time, result in relatively largeinaccuracies in the differential signals provided by base stations. InDGPS/DGNSS systems, a stationary base receiver uses its known locationas a reference for computing differential corrections that correcterrors in its own phase observations, and these corrections are thensupplied over a communication link to a rover to correct similar errorsseen at the rover. Alternatively, the base station supplies itsreference location and phase observations over the communication link tothe rover for computing the differential corrections itself, or takes amathematically equivalent approach of forming single-differences betweenbase and rover observations.

It is common practice to obtain differential position information usingbase stations to provide the additional signals to GNSS satellitepositional signals. These base stations typically use either known GNSSposition coordinates selected from a list of previously used coordinatesor automatically-generated coordinates calculated by averagingGNSS-based position data. However, when a master and multiple secondarybase stations are used for covering a large region, one base station'scoordinate system may not line up with another base station's system.Thus, when a working vehicle travels from one base station signalcoverage area to another, the positions computed by a guidance CPUwithin the working vehicle may suddenly “jump” when transitioning to thecoverage area of the new GNSS base station. The vehicle (e.g.,tractor/implement) may be guided along a guide path based on the newbase station differential positioning data which does not line up withthe previous path, resulting in uneven rows within a field.

To address this problem, Whitehead and McClure U.S. Pat. No. 7,400,294,which is assigned to a common assignee and is incorporated herein byreference, discloses portable reference stations for local differentialGPS corrections. A base or reference station location(s) is determinedand stored. Either differential correction terms or raw satellite rangesare transmitted to GNSS-equipped remote or rover vehicles for use indifferential GNSS (DGNSS) positioning and guidance operations. The baseor reference station can be removed and later returned to the generalarea of its previous location, for Which the GNSS position coordinateshave been saved. If placed within a predetermined threshold distance ofa previously-saved location, the base or reference station will “snap”to the saved reference location and compute GNSS corrector terms usingthe saved reference location data. Otherwise a new GNSS-based referenceposition will be computed for immediate use and added to the list ofstored reference locations for future use.

What is needed then is a system and method designed to coordinateseveral base stations together to form one large region of differentialguidance based off of a single “known” coordinate for a master basestation. Doing this would ensure that when the working vehicle leavesthe signal area of one base station and enters into another, the currentpath that the vehicle is traveling will not deviate or “jump” because ofa difference in the “known” coordinates that the base station is basingits differential positional information upon.

DGNSS requires rover and base GNSS receivers. The base is typicallystationary at a known location and sends to a rover GNSS receiver phase(or pseudo-range) observations plus its known location, or in lieu ofthis, differential correctors or other differential enabling data viaradios or cell phones. The rover receives the correctors from thebase(s) and uses them to correct its own satellite ranging signalobservations to increase their accuracy. The result is that the rovercan provide a more accurate location using corrected observations, evento the centimeter level or less when carrier phase is used in an RTKsolution. The range of a base signal is, however, limited. For largefields or tracts of land, a master and multiple secondary base receiversmay be needed to provide differential positioning data to the rovers tocover the entire region.

DGNSS base stations are used to generate and transmit GNSS correctionsfrom base reference positions to rover DGNSS receivers, typically byradio or cellular telecommunication, to improve accuracy to RTK or nearRTK (e.g., sub-centimeter) levels. Corrections are sent out from a basesystem, referenced to the coordinates set for the base, and eithermanually entered, automatically selected from a list of previously savedpoints, or automatically selected after a period of data averaging.

Most applications do not need absolute accuracy, but do require highrelative accuracy during field operations, both within the same week andfor subsequent field operations months or even years later. Inagriculture, this allows for planting, cultivating, and harvesting withminimal field disturbance for water and soil conservation, including theability to drive without damage to sub-surface drip tape used for highefficiency watering.

This invention allows a vehicle to base its entire preplanned path uponthe known coordinates of a single master base station, and then transferthose coordinates to calibrate other base stations as it enters thesignal area of these secondary base stations, thus expanding thesecondary base station network. Essentially, this invention willcalibrate secondary base stations to the coordinate information of asingle, master base station using a mobile transceiver device within aworking vehicle.

SUMMARY OF THE INVENTION

In the practice of an aspect of the present invention, a master basestation receiver can be either arbitrarily placed for relativepositioning or surveyed-in for absolute positioning. The master basestation references a master set of coordinates (XYZ), which can bemanually entered, automatically selected from a list of previously savedcoordinates or automatically selected after a period of data averaging.

A rover receives differential correction signals (DGNSS) from the masterbase station over a radio link for relatively precise guidance, forexample centimeter-level using real-time kinematic (RTK) techniques. Asthe rover enters the coverage area of a secondary base station, newcorrection signals are received from the secondary base station, whichcan differ substantially (e.g., or more) from the master base stationcorrection signals. Such offsets are typical with base stationsreceiving correction signals from the Wide Area Augmentation System(WAAS) and other satellite-based augmentation systems (SBAS). Theseoffsets cause an apparent GNSS-based position jump when the roverswitches to the frequency of the secondary base station upon enteringits coverage area. Merely averaging the inconsistent position correctorsfrom the master and second base stations can still result indiscrepancies of half a meter or more.

With the present invention the rover transmits these offsets over itsradio link to the secondary base station for application to itscorrection data, which will then be conformed to align with the masterbase station correction data, thus eliminating thebase-station-transition correction data jump. Correction data from themaster and the secondary base stations thus align relatively precisely,thereby enabling seamless centimeter-level guidance as the rover travelsfrom the master to the secondary base station coverage area.

A differentially-corrected-position network of base stations withoverlapping coverage polygons can thus be expanded from the originalmaster base station by computing the ΔX (latitude in degrees or meters),ΔY (longitude in degrees or meters) and ΔZ (height in meters) offsetsfrom the master coordinates for each secondary base station and applyingthese shifts at each secondary base station whereby the GNSS-basedposition discrepancies or “jumps” encountered while transitioningbetween base stations are minimized. The master-to-secondary positioninginformation shifts can either be manually entered at each secondary basestation or automatically uploaded from the rover system, e.g. as therover transitions between coverage areas around respective basestations. The rover thus functions to expand the network of overlapping,secondary base station coverage polygons as it traverses a working areaand transmits shift-based data for conforming the correctors emanatingfrom the secondary base stations.

The rover positioning system processor can define a list of basestations and their respective coverage area polygons for automaticallyswitching the rover receiver frequencies to receive the correctionsignals from the base station(s) within range. The hysteresis effectmaintains the rover receiver in contact with a current base stationuntil the rover exits its coverage area polygon, whereupon correctiondata is received from another base station, to which the rover hastransmitted the applicable master-to-secondary location coordinateoffsets over the radio link. Likewise, the hysteresis effect willmaintain the radio contact with the secondary base station until therover leaves its coverage area, whereupon coverage from the master (oranother secondary) base station seamlessly resumes.

The system can also save previously-generated secondary base locationcorrections for use in future operations, as described in U.S. Pat. No.7,400,294, which is incorporated herein by reference. For example, if asecondary base receiver antenna is returned to within a defined distance(e.g., 5 m) from a previously-generated, saved secondary base stationlocation, the secondary base location will be generated by averagingGNSS locations at that point, and corrections will be generated fromrover-transmitted master base station correction data, as describedabove. An extra message will be transmitted from the secondary basestation to the rover with the saved, previously-generated, secondarybase location corrections. Saved base station locations can thus bereused for improving rover location repeatability for accurate relativepositioning from previous DGNSS-guided operations within the coveragearea.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an aerial view of a control region with a master base GNSStransceiver and multiple secondary base GNSS transceivers each defininga respective signal coverage polygon.

FIG. 2A is an aerial view of the master base station signal coveragepolygon, showing a GNSS-equipped rover unit comprising a tractor andreceiving differential GNSS correction terms.

FIG. 2B is an aerial view showing master and secondary base stationsignal coverage polygons with the rover unit located in an overlappingcoverage area.

FIG. 2C is an aerial view showing master and secondary base stationsignal coverage polygons with the rover unit located in an overlappingcoverage area.

FIG. 2D is an aerial view showing a positioning signal shift or jumpbetween the master and secondary base station positioning signals usedfor locating the secondary base station in relation to the master basestation.

FIG. 2E is an aerial view showing the rover unit (tractor) reenteringthe master base station coverage polygon and toggling between the masterand secondary base stations for differential correction terms.

FIG. 3 is an aerial view showing the reuse of a saved secondary basestation location, including snapping to saved coordinates upon detectionwithin a predetermined radius of the saved location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Introduction and Environment

As required, detailed aspects of the disclosed subject matter aredisclosed herein; however, it is to be understood that the disclosedaspects are merely exemplary of the invention, which may be embodied invarious forms. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims and as a representative basis for teaching oneskilled in the art how to variously employ the present invention invirtually any appropriately detailed structure.

Certain terminology will be used in the following description forconvenience in reference only and will not be limiting. For example, up,down, front, back, right and left refer to the invention as orientatedin the view being referred to. Said terminology will include the wordsspecifically mentioned, derivatives thereof and words of similarmeaning.

Generally, a preferred embodiment of the present invention consists ofequipping a vehicle with a rover guidance unit and situating a number ofbase station transceiver units over an area to be traversed. One basestation will be identified as a “master” base station and should bepositioned relatively near the vehicle starting position. The basestations and rover receiver should be programmed to track the same GNSSconstellation(s), such as the global positioning system (GPS), Galileo,GLONASS, etc.

II. Portable Reference Station System 2

Referring to the drawings more detail, the reference numeral 10generally designates a portable reference station system embodying thepresent invention and generally including a master base or referencestation (base 1), multiple secondary base stations (bases 2-5) locatedin relation to the master base station 1, and a rover 11.

As shown in FIG. 1, a control region 12 generally includes multiplesignal coverage areas 14.1-14.5 forming polygons around perspective basestations 1-5 each broadcasting differential correction terms at arespective, unique frequency. Overlaps 16 are formed by two or moreoverlapping coverage areas 14.1-14.5. An obstruction 18 is shown incoverage area 14.2 and could comprise a hill, a treeline, a structure orany other signal-blocking obstruction casting a signal shadow into ablocked sub-area 20. The base stations are preferably located to providecomplete coverage of the control region with overlapping signal coverageareas whereby the rover 11 is always receiving GNSS differentialcorrection signals from at least one base station, in addition to directsatellite ranging transmissions.

FIG. 2A shows system 10 initialization with the rover 11 traversingcoverage area 14.1 and receiving GNSS-based guidance from the masterbase station 1. Without limitation on the generality of useful GNSSreceiver units, the base station 1 can include a smart antenna unit 22as shown in U.S. Patent Application Ser. No. 61/377,355, which isassigned to a common assignee herewith and is incorporated by reference.The smart antenna unit 22 includes a base antenna 23, a base GNSSreceiver 24 and a base CPU 25. Also without limitation, GNSS/RTKguidance and control systems and methods are shown in U.S. PatentPublication No. 2009/0164067, which is assigned to a common assigneeherewith and is also incorporated by reference. The system 10 applies anXYZ offset to a data stream sent from a master base station smartantenna unit 22 to the rover smart antenna unit 26, which includes arover antenna 27, a rover transceiver 28 and a rover CPU 30. This offsetcan be applied in real time either to the final position solution fromthe rover DGNSS or to the application using the rover DGNSS data. Theterms X, Y, and Z can be in distance or degrees for latitude andlongitude, and distances for altitude or any other combination ofmeasurements.

The base station 1 is preferably installed at a predetermined locationdesignated P.1, which can comprise a benchmark or monument for absolutepositioning and can be located (i.e. “surveyed in”) by conventionalsurveying techniques. For relative positioning, the location of P.11 canvary whereby the locations of the secondary base stations 2-5 areestablished relative to the location of base 1 at P.1. AdditionalGNSS-defined locations (e.g., P.2-P.4) can be established and recordedthroughout the coverage area 14.1.

The rover 11 is positioned over P.4, which can comprise a“point-of-interest” on the ground, and is located by 2D, planarorthogonal coordinates X.1, Y.1 relative to the base station 1. Thevertical axis Z coordinate at P.4 can be computed from Z.B.1 (at basestation 1) and Z.R. (at rover 11), and would be relatively constant onflat ground whereas the XY coordinates would be variable with the movingrover 11. Various GNSS-based positioning and guidance techniques can beutilized in connection with operating the rover 11, such as tiltcompensation and articulated implement guidance and control. Moreover,the system 10 enables computing the locations of the smart antenna units22, 26 as the points-of-interest, as well as ground locations formingguide paths and the locations of other objects, such as towedimplements, in fixed and variable relations to the smart antenna units22, 26.

FIG. 2B shows the rover 11 in an overlapping coverage area 16 andreceiving GNSS differential correction terms from base stations 1 and 2.The rover antennae unit 26 can toggle signal reception between the basestations 1 and 2 automatically, e.g., based on signal strength, or theoperator can manually select differential correctors from among the basestations within range of the rover 11.

FIG. 2C shows the rover 11 computing and uploading XYZ positioningsignal shifts between base 1 and base 2, Such shifts or offsets canoccur due to various contributing factors, such as satellite signalinterference, multipath, atmospheric conditions, hysteresis, etc. Therover 11 transmits updated XYZ positioning terms (calculated relative tobase 1) to base 2 for calibrating its own GNSS-defined position.GNSS-based positions can be accurately calibrated system-wide inrelation to the master base station 1 by averaging location data fromall of the base stations, as received at the rover as it moves among thecoverage areas 14.1-14.5, and from satellite-direct rangingtransmissions, as well as other real-time and stored position correctingdata sources, including public and private subscription GNSSsatellite-based augmentation systems (SBAS). Such system-wide updatingand averaging functions tend to eliminate or at least minimizepositioning signal jumps among the coverage areas. Moreover, as such XYZcoordinate shifts or offsets are detected by the rover smart antennaunit 26, they can be uploaded to suitable telecommunications links(e.g., radio or cell phone, FIG. 2C) and used as correction terms by thebase CPU 25, or used as correction terms by the rover CPU 30. Forexample, FIG. 2D shows the XYZ shift correction terms (detected at therover 11) being automatically or manually downloaded by the base station2 for calibration relative to the master base station 1.

FIG. 2E shows the rover 11 controlled by the base 2 and reentering thecoverage area of base 1, with positioning signals available from both inthe overlap 16. As described above, the rover smart antenna unit 26 canbe programmed to update the base units providing signal coverage,whereby system-wide calibration is achieved as the rover 11 travelsamong the signal coverage areas 14.1-14.5. The rover smart antenna unit26 preferably locks onto a base until its signal is lost, and thenchecks for available differential corrector signals from other bases.The rover 11 thus transitions among the base stations on-the-fly, withminimal location computation “jumps” while transitioning among bases. Ina blocked-signal subarea, such as 20, the rover smart antennae at 26 cancontinue receiving differential correctors from another base, such asbase 5 (FIG. 1), and satellite-direct ranging transmissions.

If no base station signal is picked up by the rover smart antenna unit26, GNSS guidance can continue with satellite ranging signals receiveddirectly, but accuracy will be reduced due to the lack of thedifferential positioning component. However, the system 10 will continuechecking for base station correction signals. Once a base station signalis picked up, the rover smart antenna unit 26 will guide the rover 11along the preplanned path based on the base station signal that it picksup. To reduce this drop in signal and increase the ability tosynchronize all base stations to the master base station, additionalsecondary base stations can be added throughout the field or the regionwhere vehicle guidance is occurring. This allows for a network of basestations to be easily setup using automatic averaging and networkedtogether by the application of a series of X, Y, and Z shifts.

Changing between the use of differential corrections from each basestation in view will allow the calculation of a required X, Y, and/or Zshift from the master base station 1. These shifts will be applied tothe secondary base station(s) 2-5 to make the position transitions fromusing base corrections from the master and secondary base stationsminimized. This shift can either be entered manually at each secondarybase station, or, in the preferred embodiment, automatically uploadedfrom the rover smart antenna unit 26. Using this method, additional basestations can be included within the network to create more overlappingspace, and thus reduce transitional error. Essentially, the rover 11 isacting to synchronize the secondary base station(s) 2-5 with the masterbase station 1.

The multi-base, position-averaging calibration method described above isparticularly useful in operations, such as agriculture, requiringprecision and covering regions too large for a single base station.Moreover, agricultural and other repetitive operations can benefit fromthe repeatability of the system 10 whereby the same vehicle guide pathscan be reconstructed and followed season-after-season. For example, evenif the secondary base stations were removed during off seasons, they canbe approximately relocated and the system 10 will reconfigure itselfbased on their new positions.

FIG. 3 shows a stored location snap function for reusing saved baselocations. A base smart antenna unit 22 can be placed over a newlocation P.6. The smart antenna unit 22, which is receiving GNSS rangingsignals, will compare its new computed location to a set of storedlocations. If the new location is within a predetermined distance R,such as 5 m, of a previously-saved location, the smart antenna unit 22will “snap” to the previously-saved location and use its coordinates forcalibrating an updated location using the rover-linked, multi-baseposition averaging procedures described above.

This method of an X, Y, Z offset can be made automatic by a DGNSS basestation. At each reference point occupied when base corrections aregenerated, the base location used can be saved into the CPU memory ofthe rover 11, as well as the base station's internal memory. If the basestation is returned to within a defined distance from those series ofsaved locations, such as within five meters, the base location will begenerated from averaging DGNSS locations at that point and correctionsgenerated previously. In addition, an extra message will be transmittedoccasionally that includes the X, Y, Z error from that close, previouslyoccupied base location. This shift will be applied by the rover 11 toshift the used position to utilize the reference position of thepreviously occupied base location. This allows easy reuse of baselocations and improved repeatability of vehicle travel locations givingaccurate relative positioning from previous work carried out in thearea. This is particularly useful for farm operations where repeatingpaths benefits in lower crop damage and soil erosion.

It is to be understood that while certain aspects of the disclosedsubject matter have been shown and described, the disclosed subjectmatter is not limited thereto and encompasses various other embodimentsand aspects. The above-mentioned steps and components are not meant tolimit the use or organization of the present invention. The steps forperforming the method may be performed in any logical method and theprocess can be used for other types of processes when viable.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:
 1. A positioning system fordifferentially computing a location of a rover unit with a differentialGlobal Navigation Satellite System (GNSS) including a master basestation with a master base GNSS antenna, a master base GNSS receivercoupled to the base GNSS antenna and a master base communication systemincluding a master base transmitter; and a secondary base station with asecondary base GNSS antenna, a secondary base GNSS receiver coupled tothe secondary base GNSS antenna and a secondary base communicationsystem including a secondary base transmitter; the positioning systemincluding a computer system configured to: calculate a position offsetcorresponding to an offset between differential corrections created bythe master base station and the secondary base station; transmit theposition offset to the rover unit as the rover unit transitions from acoverage area for one of the master and secondary base station to acoverage area for the other one of the master and secondary basestation; and cause the rover unit to correct a rover GNSS-definedposition with the position offset.
 2. The positioning system of claim 1,wherein the computer system is further configured to: calculatedifferent position offsets for different locations of the secondary basestation; save the different position offsets for the different locationsof the secondary base station; determine a current GNSS location for thesecondary base station; identify one of the different locations closestto the current GNSS location as a closest GNSS location; and use one ofthe saved position offsets for the closest GNSS location for thesecondary base station when a distance between the current GNSS locationand the closest GNSS location is less than a threshold distance.
 3. Thepositioning system of claim 1, wherein the computer system is furtherconfigured to transmit the position offset to the master base stationand the secondary base station.
 4. The positioning system of claim 1,wherein the computer system is further configured to: generate anaverage for position offsets between the master base station anddifferent secondary base stations; and apply the average to the roverGNSS-defined position.
 5. The positioning system of claim 1, wherein thecomputer system is further configured to: receive differentialcorrections from the master base station and the secondary base stationwhen the rover unit is located in an overlapping coverage area of themaster base station and the secondary base station; and use thedifferential corrections from the master base station to correct therover GNSS-defined position in the overlapping coverage area.
 6. Thepositioning system of claim 5, wherein the computer system is furtherconfigured to: detect a blocked-signal area within the overlappingcoverage area where signals from the master base station are blocked;and use the differential corrections from the secondary base station tocorrect the rover GNSS-defined position in the blocked-signal area. 7.The positioning system of claim 1, wherein the computer system isfurther configured to: identify a master set of location coordinates forthe master base station; identify a position offset of the secondarybase station from the master set of location coordinates; and transmitthe position offset to the secondary base station to align thedifferential corrections for the secondary base station with thedifferential corrections for the master base station.
 8. A method forusing a computing device to compute a location of a vehicle, comprising:receiving first differential correction data for a master base station;receiving second differential correction data for a secondary basestation; identifying a position shift between the first differentialcorrection data and the second differential correction data; sending theposition shift to the secondary base station to adjust the seconddifferential correction data; using the position shift to correct avehicle GNSS-defined position.
 9. The method of claim 8, furthercomprising: using the first differential correction data to correct thevehicle GNSS-defined position while the vehicle is in a first coveragearea of the master base station; and using the second differentialcorrection data to correct the vehicle GNSS-defined position while thevehicle is in a second coverage area of the secondary base station. 10.The method of claim 8, further comprising sending the position shift tothe secondary base station when the vehicle moves into an overlappingregion of the first coverage area and the second coverage area.
 11. Themethod of claim 10, further comprising: detecting a blocked-signal areawithin the overlapping region where signals from the master base stationare blocked; and using the adjusted second differential correction datafrom the secondary base station to correct the vehicle GNSS-definedposition when the vehicle is in the blocked-signal area.
 12. The methodof claim 8, further comprising deriving the position shift between thefirst differential correction data and the second differentialcorrection data relative to the master base station.
 13. The method ofclaim 8, further comprising: saving different GNSS base locations andassociated differential correction data for the secondary base station;determining a current GNSS location of the secondary base station;identifying one of the different GNSS base locations closest to thecurrent GNSS location; and use the saved differential correction datafor the identified one of the different GNSS base locations for thesecondary base station.
 14. The method of claim 8, further comprising:saving GNSS locations for the secondary base station; determining acurrent GNSS location of the secondary base station; and generatingdifferential correction data for the current GNSS location by averagingthe differential correction data for some of the saved GNSS locationsclosest to the current GNSS location.
 15. The method of claim 8, furthercomprising reusing the position shift for new locations of the secondarybase station.
 16. A positioning system for computing a location of arover unit, comprising: a computer system configured to: receive firstdifferential corrections from a first base station; receive seconddifferential corrections from a second base station; calculate aposition offset between the first differential corrections from thefirst base station and the second differential corrections from thesecond base station; send the position offset to the second base stationto calibrate the second differential corrections; and use the positionoffset to correct a vehicle GNSS-defined position.
 17. The positioningsystem of claim 16, wherein the computer system is further configured tosend the position offset to the second base station when the vehiclemoves into an overlapping coverage area of the first and second basestations.
 18. The positioning system of claim 17, wherein the computersystem is further configured to: detect a blocked-signal area within theoverlapping coverage area where signals from the first base station areblocked; and use the calibrated second differential corrections from thesecond base station to correct the vehicle GNSS-defined position whenthe vehicle is in the blocked-signal area.
 19. The positioning system ofclaim 16, wherein the computer system is further configured to calculatethe position offset between the first differential corrections and thesecond differential corrections relative to a known location of thefirst base station.
 20. The positioning system of claim 16, wherein thecomputer system is further configured to: save different GNSS baselocations and associated differential corrections for the second basestation; determine a current GNSS location of the second base station;identify one of the saved GNSS base locations closest to the currentGNSS location; and generate the differential corrections for the currentGNSS location based on the differential corrections for the closest oneof the saved GNSS base locations.
 21. The positioning system of claim20, wherein the computer system is further configured to generate thedifferential corrections for the current GNSS location by averaging thedifferential corrections for multiple saved GNSS locations closest tothe current GNSS location.